Process for the electrolytic production of opaque layers, similar to enamel, on aluminium and its alloys



Patented Aug. 22', 1944 UNITED STATES PATENT OFFICE PROCESS FOR THEELECTROLYTIC PRO- DUCTION F OPAQUE LAYERS, SIMILAR TO ENAMEL, 0N AND ITSALLOYS Carlo Sonnino and Antonio Sassetti, Milan, Italy; vested in theAlien Property Custodian No Drawing. Application August 13, 1940, Serial2 Claims.

The invention relates to a process for the electrolytic production ofopaque layers, similar to enamel, on aluminiumand its alloys, wherein acontinuous'and alternating current is used.

Many processes for producing protection layers on aluminium and itsalloys are already known, which are based on baths of anodic oxidation,containing salts of various metals, as for instance titanium. But itshould be remarked, to this purpose, that the titanium salts, astitanium oxalate, titanium phosphate, etc., under the temperature andelectrochemical operation conditions, in an oxidation bath, andparticularly in i the anodic oxidation zones, are extremely unstable,and the bath can only operate under mor mal'conditions, for a shorttime.

It is already known, that the bleaching. and the opacity of' the layerare-due to the titanium oxlwdrate' deposits, which are to be transformedinto anhydride, also by means of thermic treatments. This confines theprobability of a progressive depletion of the bath and hence the necessity of rigorous and continuous controls, in order to obtain aproduction uniformity;

According to the invention, these inconveniences are avoided and itavoids the formation of protection oxide layers, having a white-opaquecolour on aluminium and its alloys, by using the formation of suchcrystalline types of aluminium oxide, as to make itssurface entirelyopaque.

These layers are also apt to take difierent colourings when immersed incolour baths, or

In Italy October 4, 1939 i used with the addition of minimum quantitywhen a reaction of the same takes place with special chemical agents.The invention is substantially characterised by the fact, that the bathis fundamentally .con-

stituted by aqueous solutions of magnesium orzinc chromates,s eparatelyor joined with other soluble and stable salts of aforesaid metals,containing chromic acid in such sufficient quantity as to entirely keepsaid salts into solution.

It has been noted already under certain conditions, that the aqueoussolution of chromic acid. without other addition, succeeds in giving alayer of half-opaque white oxide on aluminium articles being connectedwith the positive pole of a source of continuous current and immersed inthe aforesaid bath. The magnesium and zinc salts allow the bath toalways produce such a layer of white-opaque oxide as to remove anymetallic appearance from the surface.

The layer being formed under the following mentioned conditions, is veryfire proof and does not crack as the oxide. layers formed s by theordinary processes, when the-same are submitted to a high temperature.layer is hard, very much proof against corrosion, abrasion and wear. v

The bath, according to the characteristics required for the layer, mayhave compositions comprised within the following limits:

Zinc chromate or other simple or compound (A) zinc salts: from 0.5% to20%; Chromic acid: from 5% till saturation in distilled water, or (B)Magnesium chromate, or other simple or compound magnesium "salts: from0.5% to Chromic acid: from 5% till saturation in distilled water, or

Zinc chromate or other simple or compound (C) zinc salts; from 0.5to.20%';

Magnesium chromate or other simple or compound magnesium salts: from 0.5to 20%;

Chromic acid: from 5% till saturation in distilled water.

These zinc or magnesium salts may be added together or separately, and,for certain purposes, the solution of chromic acid in water -will be(5%) of magnesium as well as of zinc salts. The

same are intended tooperate under the follow-' ing conditions oftension, current density, and temperature:

Alternating and continuous current at the tension from 10 to 25 volts; ICurrent density: from 1.8 to 2.5 ampere per square decimetre;Temperature: from 20 to 80 C.

- cuit through the solution.

Moreover. this 2. 'The process for making an anodic deposit upon anarticle of aluminium and its alloys which consists in immersing thearticle in an aqueous solution consisting essentially of chromicacidhaving a strength of between five percent and its saturation point at.20 C., fromv one-half to twenty percent of zinc chromate and fivepercent of magnesium chromate and making the article the anode of anelectric circuit through the solution. 4

CARLO SONNINO. ANTONIO 'SASSETTL'

